Synthesis, crystal structure, and thermal properties of poly[aqua(μ5-2,5-dicarboxybenzene-1,4-dicarboxylato)strontium]

In the title polymer, the eightfold-coordinated SrII atoms are linked by bridging (H2BTEC)2– anions to generate a two-dimensional network Thermal analysis (TG–SDTA–MS) has been undertaken.


Chemical context
In recent years, the self-assembly of coordination polymers (CP) and crystal engineering of metal-organic coordination frameworks have attracted great interest because of their varied molecular topologies and the potential applications of these polymers as functional materials (Pan et al., 2004;Jiang et al., 2011;Du et al., 2014). Derivatives of aromatic tetracarboxylic acids such as 1,2,4,5-benzenetetracarboxylic acid (H 4 BTEC, commonly known as pyromellitic acid) and their deprotonated forms (H n BTEC (4-n)-) belong to an important family of polycarboxylate O-donor ligands, which have been used extensively to prepare CPs (Liu et al., 2009). The variations in the possible binding modes of its four potentially coordinating carboxylic/carboxylate groups, along with the different coordination preferences of the metal ions, gives rise to a great variety of crystal structures. ISSN 2056-9890 In this communication, we report on the synthesis of [Sr(H 2 BTEC)(H 2 O)], (I), along with its characterization by single-crystal and powder X-ray diffraction, scanning electron microscopy coupled with energy-dispersive X-ray fluorescence, and thermal analysis.

Supramolecular features
In the crystal structure of (I), neighbouring layers are linked to each other along the stacking direction by intermolecular O-HÁ Á ÁO hydrogen bonds of medium-to-weak strength involving the coordinating water molecule with the carbonyl O atom (O3) of the non-coordinating carboxylic acid group as acceptor, as well as the disordered O4-H4 function of this carboxylic acid group and carboxylate O atom O4 as an acceptor group (Table 1). The hydrogen-bonding scheme is completed by two weak intermolecular C-HÁ Á ÁO interactions involving aromatic H atoms (Table 1). Based on the connectivity of these hydrogen bonds, four different motifs (Etter et al., 1990) can be distinguished, viz. R 2 2 (8), R 2 2 (10), R 2 2 (13) and R 2 2 (15) (Fig. 5), leading to a three-dimensional supramolecular structure (Figs. 6, 7). The uninodal five-connected net for (I).

Figure 5
The hydrogen-bonded-ring patterns found in (I). (a) View of the inorganic chain and (b) the two-dimensional layer structure in the crystal structure of (I).

Figure 6
View of the double-layered network along the a axis. bands located at 3164 cm À1 can be attributed to aromatic C-H stretching vibration. In addition, the symmetric [ s (OCO) = 1414 and 1346 cm À1 ] and asymmetric [ as (OCO) = 1626 and 1533 cm À1 ] stretching vibrations in (I) can be attributed to the split of the absorption bands of the carboxylate groups. The Á( ass ) values of 187-212 cm À1 indicate that some of the carboxylate groups are monodentate and bridging to the Sr II atoms. A strong absorption at 1731 cm À1 confirms the presence of the carboxylic acid function. All these results are in agreement with the crystallographic data.
Plots of the experimental and simulated powder X-ray diffraction (PXRD) patterns of the title compound are shown in Fig. 9, revealing a good match and thus phase purity and repeatable synthesis. TG/DTG, SDTA curves and the mass spectrometry analysis are depicted in Fig. 10a. TG/DTG curves of (I) reveal a total mass loss of ca 60.5% (calc. 58.1%) from room temperature up to 1273 K, with SrO as the final product. The mass loss of (I), under a dry N 2 atmosphere, proceeds in four steps. The first one, between 298 and 550 K with a mass loss of ca 5.2% (cal. 5.0%), is associated with an endothermic reaction (491 K in the SDTA curve) and corresponds to the loss of the coordinating water molecule. The second step, between 557 and 719 K with a mass loss of ca 22.1% (calc. 25.7%) and an endothermic reaction (peak at 609 K), is attributed to the beginning of the decomposition of the (H 2 BTEC) 2ligand. The third step, between 706 and 908 K with a mass loss of about 15.3% is exothermic (peak at 882 K), and may be attributed to the complete decomposition of the organic anion. The fourth step, between 908 and 1147 K with a mass loss of 17.9% is also exothermic (peak at 1121 K), and may be due to another evaporation of trapped organic moieties. The associated mass spectroscopy m/z 18 (H 2 O), 44 (CO 2 ), and 76 (C 6 H 4 ) curves (Fig. 10b) are in agreement with the TG/DTG data. The m/z 18 curve has four maxima, the first and second maxima at 565 and 639 K correspond to the loss of the coordinating water molecules. The third maximum at 682 K coincides with the m/z 44 and 76 curves, which is attributed to the first decomposition step of the organic anion, and the last maximum at 806 K coincides with the second maximum of m/z 44 and 76.     Powder XRD patterns of (I) compared with the calculated one.  (Balegroune et al., 2011). In the Ba compound, the alkaline earth cation displays a monocapped square-antiprismatic coordination environment, and the coordination mode of the (H 2 BTEC) 2ligand is monodentate to four cations at a time. The Sr compound is based on [SrO 8 ] and [SrO 9 ] polyhedra sharing edges, with the two independent (H 2 BTEC) 2ligands coordinating to five-and six-metal cations, respectively. Compound (I) with its layered structure has a different set-up and is not comparable with these two previously reported structures.

Database survey
6. Synthesis and crystallization 6.1. Synthesis Chemicals were purchased from commercial sources and used without any further purification. Compound (I) was synthesized under hydrothermal conditions. 0.26 g (1 mmol) of SrCl 2 ,6H 2 O, 0.25 g (1 mmol) of pyromellitic acid (H 4 BTEC) and 0.04 g (1 mmol) of NaOH were dissolved in water (13 ml). The reaction mixture was stirred at room temperature to homogeneity and then placed in a Teflon-lined stainless vessel (40 ml) and heated to 433 K for 3 d under autogenous pressure, and afterwards cooled to room temperature. The resulting product of plate-like single crystals and microcrystalline powder was filtered off, washed thoroughly with distilled water, and finally air-dried at room temperature.
6.2. Experimental details Powder X-ray diffraction patterns were recorded on a Philips X'pert diffractometer with Cu K radiation. The samples were gently ground in an agate mortar in order to minimize the preferred orientation. All data were collected at room temperature over the 2 angular range of 4-60 with a step of 0.01 and a counting time of 1.5 s per step. IR spectra were recorded with a JASCO FTIR-6300 spectrometer in the region 4000-600 cm À1 . SEM micrographs and X-ray microanalysis (SEM/EDX) were recorded by using a JEOL-6610LV scanning electron microscope operating at 30 kV coupled with an Oxford X-Max microanalysis system (EDX). A Mettler-Toledo TGA/SDTA851e was used for the thermal analysis in a nitrogen dynamic atmosphere (50 ml min À1 ) at a heating rate of 10 K min À1 . In this case, ca 10 mg of a powder sample were thermally treated, and blank runs were performed with the empty crucible.    (Farrugia, 2012) and DIAMOND (Brandenburg & Berndt, 2001).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound hydrogen atoms were placed in idealized positions and refined with C-H = 0.93 Å and U iso = 1.2U eq (C). The hydrogen atoms of the water molecule and of the carboxylic groups were located in a difference-Fourier map and were refined with O-H = 0.93 and 0.92 Å , respectively, and with U iso (H) = 1.5U eq (O). One of the carboxylic OH functions (O4-H4) was found to be disordered over two sets of sites of equal occupancy.

Poly[aqua(µ 5 -2,5-dicarboxybenzene-1,4-dicarboxylato)strontium]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )